On the SGU forums there was a discussion about graffiti and whether it was a good or a bad thing. In general I think graffiti can be an awesome piece of art as long as it's approved by the owner of the wall it's painted on. If it has been put up illegally I'm all against it (especially when it comes to the art deprived "tagging").
I don't know if the following is approved by the authorities or not, but it's definitely a piece of art regardless:

A wall-painted animation by BLU